Comprehensive and Detailed Explanation From Exact Extract:
A (Correct):The renderAs="application/pdf" attribute in the <apex:page> tag instructs the Visualforce page to render the content as a PDF file when accessed.
Example:
<apex:page renderAs="application/pdf">
<h1>Invoice</h1>
</apex:page>
Incorrect options:
B:renderAs="pdf" is incorrect; the correct MIME type is application/pdf.
C/D:contentType controls headers but doesn't render the content as PDF.
Reference:Visualforce Developer Guide - Generating PDFs
This is part of theUser Interface (25%)section of the exam, especially aroundVisualforce rendering and presentation logic.